1. Field of the Invention
The present invention relates to a signal transmission equipment which modulates and transmits signals accompanying clock information, such as the signals of the DS-Link (Distributed System-Link) system which is used the IEEE 1394 standard, for example.
2. Description of Related Art
In digital data transmission, it is necessary to transmit not only data signals but also clock signals for indicating timing to judge the data. To transmit clock signals, there is a method to transmit clock signals from the transmission side to the receive side along with data signals, and a method to generate clock signals at the receive side.
The DS-Link system is a method of transmitting clock signals from the transmission side to the receive side along with data signals. In the DS-Link system, two signals, that is, a data signal and strobe signal, are transmitted in parallel. To transmit such signals via radio transmission, signals must be modulated. For the modulation method, multi-value modulation, which transmits 2-bit information in one time slot (unit time), such as QPSK (Quadrature Phase Shift Keying) and 4-value FM, is usually used.
Another known method is to execute parallel/serial conversion so as to convert two signals to one signal, after which the signal is then transmitted.
Document 1 (xe2x80x9cTDMA TSUSHINxe2x80x9d, pp. 85-87, Apr. 5, 1989, issued by IEICE) disclosed a method of transmitting only data signals using multi-value modulation, and generating clock signals from data signals at the receive side. Such a method is generally used in radio transmission.
To execute the above mentioned multi-value modulation, either amplitude modulation, frequency modulation or phase modulation is used. Normally phase modulation is more frequently used since the deterioration of signals due to the influence of fading is large in amplitude modulation, and circuits become complicated if frequency modulation is used. In phase modulation, however, the positions of signal points (phase status) in a phase diagram (called a constellation) may shift, crossing the origin of the phase diagram as signals change. In a phase diagram, as FIG. 5 shows, the same phase of the carrier wave is mapped on the true axis (Ich), and the opposite phase of the carrier wave is mapped on the false axis (Qch). In the phase diagram, signal points (0, 0), (0, 1) (1, 1) and (1, 0) are positioned on the first to fourth quadrant respectively in this sequence, and the arrows in the phase diagram show the change of the signal points when the content of the signal changes. In the phase diagram, the distance between a signal point and the origin corresponds to the amplitude (power) of the signal. If a signal point shifts, crossing the origin in the phase diagram when the signal changes, this means that the signal has changed from a high amplitude status to a low amplitude status near the origin then back to a high amplitude status. Therefore, in order to accurately transmit this change of the amplitude between the transmitter and receiver, linear characteristics are demanded for both the transmitter and receiver. However, as Document 2 (xe2x80x9cKIHON DENSHI KAIROxe2x80x9d, pp. 87-96, Jan. 10, 1984, issued by the Institute of Electrical Engineers) states, a linear amplifier (class A amplifier) has poor power efficiency.
Also in the case of the method of executing parallel/serial conversion, data signals and strobe signals are actually superimposed for transmission, therefore two times the transmission speed is required. This also makes it necessary to double the width of the frequency band.
In the case of the method for generating clock signals at the receive side, on the other hand, a clock generation circuit is necessary. Also a portion called the xe2x80x9cpreamblexe2x80x9d, which is sufficient for the clock signal lead-in-time, must be added at the beginning of the transmission signal, which deteriorates the transmission efficiency. Especially when transmitting short burst signals at high-speed, the deterioration of transmission efficiency due to a preamble is a problem which cannot be ignored.
With the foregoing in view, it is an object of the present invention to provide a signal transmission equipment whereby good transmission quality can be obtained even if the linear characteristics of the transmitter/receiver are insufficient.
To achieve this object, the signal transmission equipment of the present invention has the following unique configuration. That is, the signal transmission equipment of the present invention comprises a transmitter and receiver. According to the present invention, the transmitter comprises a QPSK modulator. Also, according to the present invention, the QPSK modulator generates a modulation signal, including 2-bit digital information per unit time, which is formed based upon a first and second signal to be input. The modulation signal is output from the QPSK modulator. Also, according to the present invention, the receiver comprises a QPSK demodulator and an exclusive-or circuit. Also, according to the present invention, the QPSK demodulator converts the modulation signal sent from the transmitter to the first and second signals, and outputs therefrom the first and second signals. Also, according to the present invention, the exclusive-or circuit outputs a third signal, which is an exclusive-or of the first and second signals sent from the QPSK demodulator.
According to a preferred embodiment of the present invention, a change of the digital information which the first signal has, and a change of the digital information which the second signal has, occurs non-simultaneously.
For the embodiment of the present invention, it is preferable that the first signal is a data signal and the third signal is a clock signal.
According to another preferred embodiment of the present invention, the modulation signal is transmitted from the transmitter to the receiver by being irradiated into the air as electromagnetic waves.
According to still another preferred embodiment of the present invention, the modulation signal is transmitted from the transmitter to the receiver via optical fibers.
Another signal transmission equipment of the present invention has the following unique configuration. That is, the signal transmission equipment of the present invention comprises a transmitter and a receiver. According to the present invention, the transmitter comprises a QPSK modulator. Also, according to the present invention, the QPSK modulator generates a modulation signal, including 2-bit digital information per unit time, which is formed based upon a first and second signal to be input. The modulation signal is output from the QPSK modulator. Also, according to the present invention, the receiver comprises a QPSK demodulator. Also, according to the present invention, the QPSK demodulator converts the modulation signal, which is sent from the transmitter, to the first and second signals and outputs therefrom the first and second signals. Also, according to the present invention, in the QPSK modulator and QPSK demodulator, signal points (0, 0), (0, 1), (1, 0) and (1, 1) are positioned from the first quadrant to the fourth quadrant sequentially as binary numbers in a phase diagram, where the abscissa denotes a level of the first signal and the ordinate denotes a level of the second signal.
According to a preferred embodiment of the present invention, the first signal is a data signal and the second signal is a clock signal.
According to another preferred embodiment of the present invention, the modulation signal is transmitted from the transmitter to the receiver by being irradiated into the air as electromagnetic waves.
According to still another preferred embodiment of the present invention, the modulation signal is transmitted from the transmitter to the receiver via optical fibers.
Another signal transmission equipment of the present invention has the following unique configuration. That is, a signal transmission equipment of the present invention comprises a transmitter and receiver. According to the present invention, the transmitter comprises a QPSK modulator and a first exclusive-or circuit. Also, according to the present invention, the first exclusive-or circuit outputs therefrom a third signal which is an exclusive-or of a first and second signal to be input. Also, according to the present invention, the QPSK modulator generates a modulation signal, including 2-bit digital information per unit time, which is formed based upon the first signal and third signal sent from the first exclusive-or circuit. The modulation signal is output from the QPSK modulator. Also, according to the present invention, the receiver comprises a QPSK demodulator and a second exclusive-or circuit. Also, according to the present invention, the QPSK demodulator converts the modulation signal, which is sent from the transmitter, to the first and third signals, and outputs therefrom the first and third signals. Also, according to the present invention, the second exclusive-or circuit outputs therefrom the second signal, which is an exclusive-or of the first and third signals sent from the QPSK demodulator.
According to a preferred embodiment of the present invention, the change of a value of digital information which the first signal has, and the change of a value of digital information which the third signal has, occurs non-simultaneously.
According to another preferred embodiment of the present invention, the first signal is a data signal and the second signal is a clock signal.
According to still another preferred embodiment of the present invention, the modulation signal is transmitted from the transmitter to the receiver by being irradiated into the air as electromagnetic waves.
According to still another preferred embodiment of the present invention, the modulation signal is transmitted from the transmitter to the receiver via optical fibers.
The receiver of the present invention has the following unique configuration. That is, the receiver transmits a first and second signal therefrom, wherein the first and second signals are two kinds of digital signal which change or do not change in every time slot. According to the present invention, the first and second signals are modulated respectively to provide a first and second modulated signal, a modulation signal is generated by composing the first and second modulated signal, and the modulation signal is transmitted from the receiver. Also, according to the present invention, the first and second signals are modulated with different phases therebetween so as to invite no major amplitude change of the modulation signal over a short time.
For the embodiment of the present invention, it is preferable that the first and second signals are modulated by using QPSK modulation method.
According to a preferred embodiment of the present invention, the first signal is a data signal, and the second signal is a strobe signal which does not change in every time slot when the data signal changes in every time slot, or changes in every time slot when the data signal does not change in every time slot.
For the embodiment of the present invention, it is preferable that the first and second signals are modulated by using QPSK modulation method.
According to another preferred embodiment of the present invention, the first signal is a data signal and the second signal is an exclusive-or of the data signal and a clock signal.
For the embodiment of the present invention, it is preferable that the first and second signals are modulated by using QPSK modulation method.